The signals regulating apoptosis during the development of individual tissues are poorly understood, but a number of experimental systems has provided insights into the molecular mechanisms of several apoptotic pathways and the molecules which mediate them. Observations demonstrating that inappropriate entry into S phase is frequently associated with programmed cell death have been interpreted as indicating that these two cellular activities are coordinately regulated (3, 22, 43, 51, 73). Several genes which regulate cell-cycle progression also modulate apoptosis. Among these are the tumor suppressor proteins E2F-1, pRb, and p53 and oncoproteins such as E6, c-Jun, and c-Myc (3, 7, 22, 27, 31, 51, 55, 60, 66, 73). Evidence that the G.sub.1 checkpoint regulator p53 can cooperate with E2F to augment apoptosis, and that c-Myc and E2F-1-induced apoptosis occurs when cells are deprived of mitogenic factors (3, 22, 30) supports a model in which activation of the cell death pathway is due to a conflict between growth-promoting and growth-inhibitory signals (31, 43, 51, 73). According to this view, the tendency of cells to undergo apoptosis when receiving divergent growth signals may result from a dysregulated expression of cell cycle modulators and the unscheduled activation of their target genes.
Recent observations, however, have challenged this model by demonstrating that dual regulators of proliferation and apoptosis promote these processes by separate mechanisms (71). Active G.sub.1 cyclin/Cdk complexes are necessary for proliferation but not apoptosis mediated by c-Myc. Also, c-Myc-induced cell cycle progression and apoptosis can be separated pharmacologically (54, 64). More recently it has been demonstrated that the ability of E2F-1 to transactivate genes required for progression into S-phase can be uncoupled from its ability to activate apoptosis (9, 33, 58). Similar observations have been reported for the p53 tumor suppressor whose ability to induce growth arrest and apoptosis are genetically separable (9). These findings support the notion that, while several regulators of the cell death and growth pathways are shared, their functional activities in these pathways are distinct. How these divergent activities are coordinated remains unknown.
Id proteins comprise a family of helix-loop-helix (/BLH) transcription factors that are important regulators of cellular differentiation and proliferation (5, 6, 11, 21, 61, 69, 75). Id proteins lack a basic DNA binding region and are capable of inhibiting gene expression by forming inactive heterodimers with bHLH transcription factors, thereby blocking the binding of bHLH factors to specific DNA sequences, E-boxes, found in the regulatory regions of their target genes (11, 40, 48, 52, 56, 69). During cellular proliferation and prior to the onset of differentiation, Id genes are typically expressed at high levels (5, 19, 35, 36, 42, 69) and enforced overexpression of Id genes inhibits differentiation of a variety of cell lineages (13, 18, 38, 42, 47, 50, 67, 68). In addition, constitutive expression of Id1 in B-cells of transgenic mice inhibits B-cell maturation during the establishment of the immune system (68).
Id gene expression is enhanced in response to mitogenic stimuli (4, 5, 11, 16) and has been implicated in the induction of DNA synthesis (57). Ectopic expression of Id1 or Id2 enhances proliferation of several cell types (34, 59). Furthermore, treatment of cells with antisense Id1 oligonucleotides or microinjection of Id1 neutralizing antibodies prevents re-entry of serum deprived cells into the cell-cycle (4, 28). Some aspects of the mechanisms by which Id proteins enhance proliferation have been described: Id2 binds to the unphosphorylated retinoblastoma protein (pRb) family members and abolishes their growth-suppressing function (34, 45), and Id1a inhibits expression of p21/WAF1, an inhibitor of cyclin dependent kinases (59). Moreover, Id2 can antagonize the growth inhibition of several tumor suppressor proteins whose action is mediated by pRb including p21 and p16 (45). Other components of the cell cycle machinery interact with Id2 as well. Recently, Id2 was shown to be an in vivo substrate of the G.sub.1 -cyclin-dependent kinases, cyclin A- and cyclin E/Cdk2 (29). The only other substrates identified for these kinases are the Rb family of tumor suppressors, pRb, p107 and p130, and certain E2F family members, all of which are important for the G.sub.O to S-phase transition (for a review see 72).
We have found that intracellular overexpression of Id2 N-terminal domain containing polypeptides can modulate apoptosis. To date, all Id protein functions have been ascribed to the HLH sequence motif, which mediates heterodimerization with b/HLH transcription factors (5) and the association of Id2 with Rb family members (34, 45). Furthermore, the ability of certain bHLH transcription factors to induce apoptosis has been assigned exclusively to their HLH//LZ dimerization domains (Kohluber et al., 1995, J.Biol.Chem. 270, 28797-28805). Strikingly, Id2 transmits an apoptotic death signal that is independent of its ability to dimerize with bHLH members. Since the Id2 HLH domain is required for interactions with the pRB family (34, 45), the cell death and growth promoting activities of Id2 are separable. We found that the apoptotic activity of the N-terminal region of Id2 is associated with the expression of a known modulator of the programmed cell death cascade, Bax, indicating a role for Id2 in integrating the divergent cellular activities of growth and apoptosis.
Cited Literature
1. Almasan, A., et al. 1995. Proc. Natl. Acad. Sci. 92: 5436-5440. PA0 2. Andres-Barquin P. J., et al. 1997. Cancer Res. 57: 215-220. PA0 3. Askew, D. S., et al. 1991. Oncogene 6: 1915-1922. PA0 4. Barone, M. V., et al. 1994. Proc. Natl. Acad. Sci. 91: 4985-4988. PA0 5. Benezra, R., et al. 1990. Cell 61: 40-61. PA0 6. Biggs, J. R., et al. 1992. Proc. Natl. Acad. Sci. 89: 1512-1516. PA0 7. Bossy-Wetzel E., L. Bakiri, and M. Yaniv. 1997. EMBO J. 16: 1695-1709. PA0 8. Chen, B., et al. 1997. Nucleic Acids Res. 25: 423-430. PA0 9. Chen, X, et al.. 1996. Genes & Dev. 10: 2438-2451. PA0 11. Christy, B. A, et al. 1991. Proc. Natl. Acad. Sci. 88: 1815-1819. PA0 12. Cleveland, J. L., et al. 1994. Oncogene 9: 2217-2226. PA0 13. Cordle, S. R., et al. 1991. Mol. Cell. Biol. 11: 1734-1738. PA0 14. Condorelli, G. L., et al. 1997. Mol. Cell. Biol. 17: 2954-2969. PA0 15. Cubas, P., et al. 1994.. Development 120: 2555-2566. PA0 16. Deed, R. W., et al. 1993. Oncogene 8: 599-607. PA0 17. Deed, R. W., et al. 1996. J. Biol. Chem. 271:23603-23606. PA0 18. Desprez, P. Y., et al. 1995. Mol. Cell. Biol. 15:3398-3404. PA0 19. Einarson M. B. and M. V. Chao. 1995. Mol. Cell. Biol. 15:4175-4183. PA0 20. Ellis, H. M., D. R. Spann, D. R. and J. W. Posakony. 1990. Cell 61:27-38. PA0 21. Ellmeier, W., et al. 1992. EMBO J. 11:2563-2571. PA0 22. Evan, G. I., et al. 1992. Cell 69:119-128. PA0 23. Fanidi, A., E. A. Harrington, and G. I. Evan. 1992. Nature 359:554-556. PA0 24. Garrell, J. and J. Modolell. 1990. Cell 61:39-48. PA0 25. Gavrieli Y., et al.. 1992. J. Cell Biol. 119:493-501. PA0 26. Gossen, M., et al. 1995. Science 268:1766-1769. PA0 27. Haas-Kogan, D., et al. 1995. EMBO J. 14:461-472. PA0 28. Hara, E., et al. 1994. Biol. Chem. 269:2139-2145. PA0 29. Hara, E., M. Hall, and G. Peters. 1997. EMBO J. 16:332-342. PA0 30. Harrington, E. A., et al. 1994. EMBO J. 13:3286-3295. PA0 31. Hiebert, S. W., et al. 1995. Mol. Cell Biol. 15:6864-6874. PA0 32. Hinds P. W. and R. A. Weinberg. 1994. Curr. Opin. Genet. Devel. 4:135-141. PA0 33. Hsieh J. K., et al. 1997. Genes & Dev. 11:1840-1852. PA0 34. Iavarone, A., et al. 1994. Genes & Dev. 8:1270-1284. PA0 35. Ishiguro, A., et al.. 1996.. Blood 87:5225-5231. PA0 36. Ishiguro, A., et al. 1995. Leuk. Res. 19:989-996. PA0 37. Jen, Y., K. Manova, and R. Benezra. 1996. Dev. Dynam. 207: 235-252. PA0 38. Jen, Y., H. Weintraub, and R. Benezra. 1992. Genes & Dev. 6: 1466-1479. PA0 39. Jen, Y., K. Manova, and R. Benezra. 1997. Dev. Dynam. 208:92-106. PA0 40. Kadesch, T. 1993. Cell Growth Differ. 4:49-55. PA0 41. Kowalik, T. F., et al. 1995. J. Virol. 69:2491-2500. PA0 42. Kreider, B., et al. 1992. Science 255:1700-1702. PA0 43. Krek, W., G. Xu, and D. Livingston. 1995. Cell 83:1149-1158. PA0 44. Kurabayashi M., R. Jeyaseelan, and L. Kedes. 1993. Gene 133:305-306. PA0 45. Lasorella, A., et al.. 1996. Mol. Cell. Biol. 16:2570-2578. PA0 46. Le Jossic C., et al. 1994. Cancer Res. 54: 6065-6068. PA0 47. Lister, J., et al. 1995. J. Biol. Chem. 270:17939-17946. PA0 48. Littlewood, T. D. and G. I. Evan. 1995. Protein Profile 2:621-702. PA0 49. McLeod, K. F., Y. Hu, and T. Jacks. 1996. EMBO J. 15:6178-6188. PA0 50. Moldes, M., et al. 1997. Mol. Cell. Biol. 17:1796-1804. PA0 51. Morgenbesser, S. D., et al. 1994. Nature 371:72-74. PA0 52. Murre, C., P. S. McCaw, and D. Baltimore. 1989. Cell 56: 777-783. PA0 53. Nicoletti, I., et al. 1991. J. Immunol. Methods 139:271-279. PA0 54. Packham, G., C. Porter, and J. L. Cleveland. 1996. Oncogene 13:461-469. PA0 55. Pan, H. and A. E. Griep. 1994. Genes & Dev. 8:1285-1299. PA0 56. Pesce, S. and R. Benezra. 1993. Mol. Cell. Biol. 13:7874-7880. PA0 57. Peverali, F. A., et al. 1994. EMBO J. 13:4291-4301. PA0 58. Phillips, A. C., et al. 1997. Genes & Dev. 11:1853-1863. PA0 59. Prabhu, S., et al. 1997. Mol. Cell. Biol. 17:5888-5895. PA0 60. Qin, X. Q., et al. 1994. Proc. Natl. Acad. Sci. 91:10918-10922. PA0 61. Riechmann, V., et al. 1994. Nucleic Acids Res. 22:749-755. PA0 62. Rodriguez-Tarduchy, G. M., et al.. 1990. EMBO J. 9:2997-3002. PA0 63. Rowan, S., et al. 1996. EMBO J. 15:827-838. PA0 64. Rudolph, B., et al. 1996. EMBO J. 15:3065-3076. PA0 65. Schmitz, G. G., et al. 1991.. Anal. Biochem. 192:222-231. PA0 66. Shan, B. and W. H. Lee. 1994. Mol. Cell. Biol. 14:8166-8173. PA0 67. Shoji, W., et al. 1994. J. Biol. Chem. 269:5078-5084. PA0 68 . Sun X. -H. 1994. Cell 79:893-900. PA0 69. Sun, X. -H., et al. 1991. Mol. Cell. Biol. 11:5603-5611. PA0 70. Steller, H. 1995. Science 267:1445-1449. PA0 71. Wagner, A. J., et al. 1994. Genes & Dev. 8:2817-2830. PA0 72. Weinberg, R. A. 1995. Cell 81:323:330. PA0 73. Wu, X. W. and A. J. Levine. 1994. Proc. Natl. Acad. Sci. 91:3602-3606. PA0 74. Zacksenhaus, E., et al. 1996. Genes & Dev. 10:3051-3064. PA0 75. Zhu W., et al.. 1995. Molecular Brain Research 30:312-326.